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SN74HCS244-Q1: Jtag demultiplexer for 1~3m cable

Part Number: SN74HCS244-Q1
Other Parts Discussed in Thread: TMUX1308, , SN74LVC125A, SN74LVC3G17, SN74ABT541B-Q1

Hi:

I'd like to design 8 channels demultiplexer for JTAG (Jtag repeater).

Front-end use 4*TMUX1308 as demultiplexer.

(TDI, TDO, TCK, TMS)

TCK work on 75 MHz.

Now,

I need to pick up buffer for 2 meter jtag cables.

SN74HCS244-Q1 seems fine,

but I'm not sure it can support 2m cables,

or could I have better solutions?

Thanks!

Best Regards!

  • Long cables need high drive strength. HCS might not have enough, especially at voltages below 5 V.

    I'd suggest LVC, i.e., something like SN74LVC3G17 for 5 V, or SN74LVC125A for 3.3 V.

    How many channels at what voltage? Do you need OE?

  • Hi @

    Thanks for your reply,

    No need for OE.

    I also prefer to "No additional buffer in the end of cables",

    so I'd like to pick higher strength buffer.

    Do you have other better options?

    And I also have a problem.

    Which parameter in the specification determines whether it is suitable for long cables?

  • Better options would use differential signals (e.g., LVDS), which would require two lines and a receiver.

    Suitability is determined mostly by drive strength (IOL, IOH).

  • Hi @ Guru :

    Thanks for your reply.

    This idea is since we need programming lots of FPGA (~100 pic) with one JTAG programmer.

    If we switch programming each boards manually, then it is not efficiency.

    So we'd like to make a switch board,

    and controlled sel pin with MCU to switch FPGA board for programming.

    Since FPGA board and its JTAG pings (single-end) has been made,

    so we only use single-end buffering JTAG singles.

    I also find ABT series (SN74ABT541B-Q1),

    IOL/IOH is higher than LVC series.

    It seems a good options.

  • Hi DeWei,

    I think you'll find that you get a large amount of ringing at the JTAG receiver with 1-3m of cable, pretty much regardless of what driver you use.

    If I were building this system, I would have the programmer built such that it sends the same data across all cables via LVDS (or a similar differential protocol), then at the connector it would be converted to single-ended signals for JTAG. This will increase cost slightly for the programming station, but since it's a single unit basically, it should be marginal compared to trying to implement some sort of receiver solution on every device to be programmed.